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I have a water tank on a hill and some lawns to water at the bottom of the hill.

The pipe that connects those two is a 2.5" pipe (yes, really) and when I attach a pressure gauge at the bottom of this 2.5" pipe I see a pressure of about 35 PSI, but when I connect the water tank, the head will be another 10 feet higher (it's a 10 foot tall tank) so ... figure 40 PSI.

I think my flow rate in GPM is extremely high. I tried some calculators online but am not sure if I am getting it right but it appears that I have hundreds and hundreds of GPM. And again, the pressure is 35-40 PSI.

It would be really nice to not have to purchase, install, and maintain a pump for this application, and it seems like I would not need one ... but may I sanity check:

If I connect a sprinkler zone at the bottom, feeding in 35-40 PSI at (god knows what huge GPM) and that sprinkler zone is maybe 200 ft. of 3/4" PVC and say 4 impact sprinkler heads that are designed to run at 30 PSI ... good to go ?

I lose 1-2 PSI on the 200 ft. of 3/4 PVC run ... do I lose any PSI on each of the impact heads (they are in a series) ?

Am I missing something here ?

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  • More information - the sprinklers I am using are the rain bird 2045A Maxi-Paw which can operate anywhere from 25-60 PSI, depending on which nozzle plug you use ... I also have some 35ADJ-TNT-B 3/4 brass impact heads that the spec sheet says can run at 30-60 PSI (with 44 - 51 foot radius, depending...)
    – user227963
    Apr 8, 2019 at 7:03
  • More information goes in your question, not comments. Thanks.
    – isherwood
    Jan 3, 2020 at 15:29

2 Answers 2

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You certainly do not need a pump. Here's the secret to getting enough water to sprinklers. You don't need to run them all at the same time.

So if you have 4 sprinklers and only have the flow to run 2 of them happily, then simply have 2 zones. Run one until it's done. Then run the other.

Best to do it at night - you'll lose less water to evaporation.

Another trick you can do is reduce (ratchet back) the run-time of the sprinklers. Lots of people vastly, vastly over-water their lawns. Reduce the runtime by 15% a week until the lawn starts to show distress... then increase it back up one notch.

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From the position of the sprinklers you have 70 feet of head. You have shown that you have sufficient pressure.

What you need to consider is that if you have too many sprinklers then the pressure drops and so does the performance.

You will only need a pump if the area to be irrigated is too large and you need too many sprinklers....

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  • From the research I have done, each individual sprinkler head represents a cumulative drop in FLOW (gpm) but I don't see anyone specifying a pressure drop per head. I feel intuitively that if I have four heads in a row (in a series - not on their own stubs) that pressure PSI must drop after each head but I am having trouble verifying that intuition ... FWIW, each zone will only have 4-6 sprinklers so perhaps I have so few that it won't matter, given my HUGE flow rate ?
    – user227963
    Apr 8, 2019 at 16:14
  • BTW, the reason I think this pipe has a very high flow rate is that I looked at the table on hy-techroofdrains.com/water-flow-through-a-pipe and I see that 2.5" pipe @ 40 psi is 421 GPM ... does that still hold true if pressure is 0psi at the top of the pipe and 40 at the bottom ? Am I misjudging this rough calculation somehow ?
    – user227963
    Apr 8, 2019 at 16:24
  • The inlet pressure changes, have a look at this calculator : engineeringtoolbox.com/flow-liquid-water-tank-d_1753.html
    – Solar Mike
    Apr 8, 2019 at 16:47
  • Can you elaborate ? I am not sure which question or random thought of mine above you were referring to ... Thank you.
    – user227963
    Apr 9, 2019 at 5:22
  • Why not try the calculator with your tank and pipe information...
    – Solar Mike
    Apr 9, 2019 at 5:30

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